First ray hypermobility is implicated in many forefoot pathologies, yet its quantitative assessment remains challenging. Instrumented methods have traditionally focused on isolated dorsal displacement of the first metatarsal, which may not reflect physiological load sharing within the forefoot. This cadaveric biomechanical study evaluated the reliability and construct validity of two arthrometer-based measures: the first ray absolute (FRAM) and relative (FRRM) mobility, respectively assessed under isolated loading of the first metatarsal and symmetrical loading of the first metatarsal and lesser metatarsals. Ten fresh-frozen cadaveric lower limb segments were tested using an automated forefoot arthrometer. Within-session reliability was quantified using intraclass correlation coefficients, standard error of measurement, and minimal detectable change. Construct validity was assessed by correlating FRAM and FRRM with a biomechanical reference construct defined as the sum of superior-inferior translations at the first tarsometatarsal and medial naviculo-cuneiform joints measured using optical motion capture. Linear mixed-effects models were used to characterise joint-level kinematic behaviour under each loading mode. FRRM demonstrated excellent within-session reliability (ICC=1.00) and a strong correlation with the reference construct (R=0.84, p=0.003). FRAM also showed excellent reliability (ICC=0.97) but higher absolute measurement error and a weaker, non-significant association with the reference construct (R=0.62, p=0.057). Symmetric loading engaged proximal first ray joints more effectively than isolated loading, which predominantly mobilised the first tarsometatarsal joint. These findings indicate that arthrometer-based assessment of the first ray mobility is sensitive to the loading mode and that symmetric loading provides a more biomechanically representative evaluation of the first ray mobility than traditional isolated approaches.